Application of local mechanical tensioning and laser processing to improve structural integrity of multi-pass welds

Abstract:

Multi-pass fusion welding by a filler wire (welding electrode) is normally carried
out to join thick steel sections used in most engineering applications. Welded
joints in an installation, is the area of critical importance, since they are likely to
contain a higher density of defects than the parent metal and their physical
properties can differ significantly from the parent metal. Fusion arc welding
process relies on intense local heating at a joint where a certain amount of the
parent metal is melted and fused with additional metal from the filler wire. The
intense local heating causes severe transient thermal gradients in the welded
component and the resulting uneven cooling that follows produces a variably
distributed residual stress field. In multi-pass welds, multiple thermal cycles
resulted in a variably distribution of residual stress field across the weld and
through the thickness.
These complex thermal stresses generated in welds are undesirable but
inevitable during fusion welding. Presence of such tensile residual stresses can
be detrimental to the service integrity of a welded structure. In addition to a
complex distribution of residual stress state, multi-pass welds also forms dendritic
grain structure, which are repeatedly heated, resulting in segregation of alloying
elements. Dendritic grain structure is weaker and segregation of alloying
elements would result in formation of corrosion microcells as well as reduction in
overall corrosion prevention due to depletion of alloying elements.